A sucker rod is a steel rod, typically between 25 and 30 feet long, and threaded at both ends, used in the oil industry to join together the surface and downhole components of a reciprocating piston pump installed in an oil well. A pumpjack is the visible above-ground (surface unit) drive for the well pump, and is connected to the downhole pump at the bottom of the well by a series of interconnected sucker rods.
The surface unit transfers energy for pumping the well from a prime-mover to a sucker rod string. In doing this, it must change the rotary motion of the prime-mover to a reciprocating motion of the sucker rod. Also, it must reduce the speed of the prime-mover to a suitable pumping speed. Speed reduction is accomplished by a gear reducer. Rotary motion of a crank shaft is converted to an oscillatory motion by a walking beam. A crank arm is connected to the walking beam by a pitman arm. The walking beam is supported by a Samson post and a saddle bearing. A horse head and bridle are used to ensure that the pull on the sucker rod string is vertical all times so that no bearing movement is applied to that part of the sucker rod string above stuffing box. The polished rod and stuffing box combination is used to maintain a good liquid seal at the surface.